Defect and grain boundary engineering for enhanced performances and lifetimes of hybrid perovskite solar cells
As the world energy consumption increases, the necessity for clean energy proves vital and urgent as catastrophic effects of global climate change are imminent. The sun delivers more energy to Earth in just over an hour than we have consumed over the course of last year, making solar technology a most promising candidate for a sustainable future. Currently, state-of-the-art silicon-based solar technologies dominate the market owed to their maturity in processing knowledge, performance reliability, and lifetime. Despite lower costs in recent years, the levelized cost of electricity of silicon-based solar PV technologies is still much greater than conventional fossil fuel sources, providing large incentive to find more cost-effective alternative solar PV technologies. Solution-processed solar technologies are appealing as a low-cost alternative, among which, hybrid perovskites have recently risen as a high performance solution-processed PV technology achieving laboratory scale efficiencies >23%, which are rapidly approaching those of conventional Si-based PV (~26%). However, owed to the solution-processed nature of perovskites, there are many associated defects and grain boundary regions that can adversely affect performances and lifetimes. In this dissertation, I will focus on addressing defect and grain boundary engineering, taking advantage of the wet chemical environment of perovskite thin film processing to manipulate crystallization dynamics and modulate defect and grain boundary properties.
In Chapter 4, the guanidinium molecule was first discovered for use in hybrid perovskites. It was found that inclusion of guanidinium can provide extraordinary enhancements in photoluminescent properties of the thin film and enhanced open-circuit voltage of the devices. Based on further experimental analyses and results, we believe that the guanidinium ion serves to suppress formation of defects via lattice strain relaxation and also may serve as a passivant at grain boundaries, giving rise to these impressive improvements. This work opened the door to future works throughout my Ph.D. to control crystallization and defect properties.
To further investigate the nature of controlling crystal growth and associated defect natures, a Lewis acid-base adduct approach was developed and is discussed in Chapter 5. The strength of interactios between the Lewis acid perovskite precursors and Lewis base additives were shown to greatly influence nucleation and growth dynamics. The Lewis base urea was found to provide great enhancement in crystal growth, producing larger sized grains and enhanced photoluminescent properties, alongside a greater performance and stability of the device.
Lastly, in Chapter 6, the intrinsic stability of the perovskite thin film is addressed by utilizing the compositional and dimensional tenability of hybrid perovskite materials, utilizing a hybrid 3- and 2-dimensionality of the perovskite structure to lead to even further enhanced performances and long-term lifetimes. It was found that the 2-D perovskite forms around grains (at grain boundaries) to facilitate the production of high quality 3-D grains and serve as passivating layers to grain boundaries. These works provide important future directions for hybrid perovskite PV research to realize commercial technologies with competitive efficiencies, long lifetimes, and low costs. Conclusions and future outlooks of these works are discussed in Chapter 7.